Process of treating superphosphates



June

M. SHOELD 2,288,112

PROCESS OF TREATING SUPERPHOSPHATES Filed Feb. 12, 1940 SUPERPHOSPHATE CALCINED IN FLOW OF GASES TO A REDUCE FLUORINE CONTENT GRIND THE CALCINED SUPERPHOSPHATE CONQENTRATED SULPHURIC ACID THOROUGH MIXING IN MECHANICAL MIxER HEATING AGIDULATED MATERIAL TO TEMPERATURE OF INCIPIENT S0 FUMES PHOSPHATE MATERIAL OF H'IGH'NEUTRALIZING VALUE AND EXTREMELY LOW FLuoRINEcoNTENT WWW M 09 i m Patented June 30, 1942 PROCESS OF TREATING SUPERPHOS- PHATES Mark Shoeld, Baltimore, .Md., assignor to The Davison Chemical Corporation, Baltimore, Md.

Application February 12, 1940, Serial No. 318,595

6 Claims.

My invention relates to a process for the manufacture of phosphatic material which is nontoxic and of a low fluorine content.

This invention is an improvement on the process and product disclosed in Anderson and Wight Application Serial No. 194,198 filed March 5, 1938, now Patent No. 2,234,511.

In general, my invention consists in improving the process and product disclosed in the above-identified application. In'that application superphosphate, which may or may. not be of granular form, is subjected to a treatment to reduce the fluorine content. In case granular superphosphate is employed, the form of the material may be such that each granule has acrushing strength of several times the ordinary den superphosphate and the uncrushed granules are globoid, induratedand encrusted particles. I may also employ besides the granular superphosphate and the ordinary den superphosphate the product which is obtained by crushing granular superphosphate, or my starting material may be a mixture of ordinary den superphosphate and either crushed orv uncrushed granular superphosphate.

The product which I derive from treating the granular superphosphate is in some respects superior to that obtained when other starting materials are employed. Accord ng to the process set forth in the above-identified application, Se-

rial No. 194,198, the starting material is caused to pass through a calcining zone where it is subjected to a flow of hot gases and to a temperature of from 600 to 800 C. The material is ordinarily agitated when passed through the heating zone by tumbling it in a rotary drum in which the material moves progressively from one end of the drum to the other. There is ordinarily a counter-current flow of the heated gases through the rotating drum. The fluorine content in this calcination step in the rotary drum is reduced below 0.1%, and the P205 content is rendered soluble in a 4% HCl to the extent of approximately 98% of such P205 content. Though the fluorine content by this calcining process generally is reduced below 0.l%, I do not wish to be limited to the exact percentages herein set forth, which are given merely by way of illustration.

This calcined superphosphate is then ground and concentrated sulphuric acid is added, the

two being mixed thoroughly in a mechanical mixer. The mixture is then heated to a temperature of incipient SO: fumes, that is to say, a temperature at which SO: fumes areabout to be given off.

This process gives a phosphatic material of I high neutralizing value and extremely low fluorine content. The material is non-toxic, free flowing, non-hygroscopic and stable.

One object of my invention is to produce a solid, non-toxic, acidic, phosphatic material which is free flowing, non-hygroscopic and stable.

Another object of this invention is to reduce the fluorine content of this phosphatic material to a very low percentage.

Yet another object is the production of a material that has a neutralizing value, 1. e., a product that is sufiiciently acidic to release carbon dioxide from sodium bicarbonate. This property of my material is of particular use in leav- ---ening baked food products, such as, bread, cake,

etc.

Still another object of the invention is the production of an extremely low fluorine content material which may serve as a basis for the production of pure phosphatic compounds.

In the drawing:

Fig. 1 represents a flow sheet indicating in general the steps of my process starting with the calcined superphosphate.

Fig. 2 is an analytical table showing the changes which occur in the original material and its characteristics at the end of the calcination step and in its final form. This showing is of one example only and is merely illustrative.

As previously indicated, the starting superphosphate material is subjected to a tumbling action in a rotary drum in which material is fed at one end of the drum and passes through to the other end. A counter-current flow of hot gases is maintained in the interior of the drum. The calcining temperature is around 600 to 800 Cf No fusion whatever takes place in this calcining step. As previously pointed out, granular superphosphate is particularly suitable for this operation because of the uniformity of the particle size and resultant uniform heating. In addition to this advantage, the use of granular superphosphate eliminates troubles due to dust losses. This, from a practical point of view, is of great importance.

There is no oflicial method for the determination of the digestibility of phosphate compounds used for feeding purposes. A method used in the trade, however, is the determination of solubility in a water solution of 0.4% HCl. The reason for this is that this is the acid and approximate strength of the acidity of the digestive juices. The analytical method involves digesting one gram sample in 100 c. c. at 65 C. for one hour.

By means of this I-ICl test as well as by feeding experiments our defluorinated phosphate from the calcination step shows practically complete availability, the insoluble P205 after calcination being only about 0.4% out of a total of 27.2%; or 1.47% of the total P205. Another way of stating this is to say that 98.53% of the total P205 is soluble. It is necessary to have a current of gas in this calcination step. If the superphosphate is calcined in the absence .of gas currents as for instance in an electric oven, as much as one-fourth of the total P205 changes to insoluble compounds, possible meta-phosphates, that do not show up as P205 by the ofliemploy 98% acid. The acid and ground materialis very thoroughly incorporated using either a pan mixer, or a dough mixer, or some other suitable mechanical mixer.

While I prefer to employ an acid of the concentration above mentioned, I have found that acids of a concentration as low as 65 to 70% sulphuric acids work very satisfactorily.

The thoroughly mixed material which is still substantially solid is passed through a heat treating operation. The heating should be carried to the point where S05 fumes start to be given off, that is to say, heating to incipient S03 fumes. I have found that this heating step generally involves heating the material to a temperature of approximately 250 to 350 C. for a period of about five to ten minutes.

I would like to point out that although the above conditions regarding temperature and time is the preferred mode of operation, lower temperatures can be employed, and in such cases the time of the heat treatment is prolonged. For example, the material can be heated to from 150 to 200 C. for a period of from one-half hour to one hour. In addition, the temperature can be as low as 100' C. and in such a case it is necessary to heat the material over night or for a period of approximately twelve hours to obtain satisfactory results. In case of these lower temperatures there is of course no incipient S03 fumes. In the lower temperatures above mentioned the non-hygroscopic and free flowing properties of the product are considerably improved although they are not quite as satisfactory as those obtained according to the preferred mode of operation.

Prior to heat treatment the ground calcined superphosphate when acidulated with the con centrated sulphuric acid is a more or less hygroscopic material. When put in a closed container it develops a very strong fluorine smell. The heat treating of the product eliminates almost all of the fluorine and makes the material nonhygroscopic and free flowing. Exactly what happens, I do not know, but I do know that the material has a little more than a trace of fluorine in it.

The theory of the fluorine elimination in the original superphosphate up through the calcination step however follows. wish to be bound by the theory herein given,

but I believe the reactions to be as set forth. I

believe that in the calcination step the monocalcium phosphate of the superphosphate is sufflciently acidic at the calcining temperature to drive ofi a considerable amount of S0: from the calcium sulphate. Calcium sulphate itself at the temperature used in the calcining operation would not, of course, dissociate at all or lose any S03. Thus the marked fluorine elimination in the calcining operation is no doubt due to the fact that superphosphate is a mixture of monocalcium phosphate and calciumsulphate. The

S0: given off chases the fluorine out of the mix..

This is possible in view of the fact that no fusing or fiuxing of the material in the caicination step occurs, either of which would have a tendency to reduce the elimination of fluorine. I have found that more than 30% of the SO: occurring in the calcium sulphate in the original superphosphate is driven off by the calcination. This can be easily computed, of course, from the fact that the P205 is, of course, non-volatile during the calcination, and by comparing the P205, S03 ratio in the original and calcined product, the amount of S03 driven of! can be readily computed.

' Referring to Fig. 2, in the column at the left I show diiierent compounds entering into the make-up of the material. The abbreviation N. V. stands for the neutralizing value of the product or, in other words, the parts by weight of sodium bicarbonate from which the 00-; will the original material the P205 was 22.0% and the S03 percent was 31.7%. Thus, as before indicated, more than 30% of the S0: bound as CaSOr in the original superphosphate has been driven oil. It will be further noted that the fluorine content of the original granulated superphosphate which was 1.4% was reduced to 0.09% at the end of the calcining operation, and that by the process of this invention the fluorine ,content was reduced to 0.015%, after complete processing.

Not only is the fluorine content in the product low, but it is non-toxic, non-hygroscopic, stable and free flowing.

The data given in the table oi. Fig. 2 is intended to be merely illustrative of one sample material, and I do not intend to be bound by the exact figures given, but only by the showing of the'prior art and the scope of the appended claims. 4

I claim:

1. A process of preparing an acidic, non-hygroscopic, free-flowing superphosphate having a low fluorine content and a high neutralizing value comprising the steps of calcining a superphosphate in the presence of heated gas cur- I, of course, do not rents between 600 to 800 0.; grinding the calcined superphosphate, reacidulating the calcined superphosphate with a substantial amount of concentrated sulphuric acid and thereafter heating the reacidulated calcined superphosphate for a period of time and at a temperature suflicient to reduce the fluorine content to about 0.015%.

2. A process of preparing an acidic, non-hygroscopic, free-flowing superphosphate having a low fluorine content and a high neutralizing value as set forthin claim 1 wherein the reacidulated calcined superphosphate is heated to incipient SO: fumes.

3. A process of preparing an acidic, non-hygroscopic, free-flowing superphosphate having a low fluorine content and a high neutralizing value comprising the steps of calcining a superphosphate in the presenceof heated gas currents between 600 to 800 C., grinding the calcined superphosphate, mixing the ground calcined superphosphate with concentrated sulphuric acid in the ratio of about 100 parts of the superphosphate to about 35 parts by weight of the sulphuric acid and thereafter heating the mixture of ground calcined superphosphate for a period of time and at a temperature suflicient to reduce the fluorine content to about 0.015%.

4. A process of preparing an acidic, non-hygroscopic, free-flowing superphosphate having 8.

mately twelve hours at a temperature of about 6. A process of preparing an acidic, non-hygroscopic, tree-flowing superphosphate having a low fluorine content and a high neutralizing value comprising the steps of calcining a superphosphate in the presence of heated gas currents between 600 to 800 C., grinding the calcined superphosphate, mixing the ground calcined superphosphate with concentrated sulphuric acid in the ratio of about 100 parts of superphosphate to 35 parts by weight of sulphuric acid and thereafter heating the mixture of ground calcined superphosphate and sulphuric acid for a period of from about five to ten minutes at atemperature of about 250 to 350 C. to reduce the fluorine content to about 0.015%.

, MARK SHOELD. 

